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Fluorescent materials convert some visible or invisible electromagnetic radiations from a certain range of frequencies to another one, usually from higher to lower frequencies, rarely, the other way around ("anti-Stokes"). Is there any equivalent mechanism in acoustics? Are there any structures that are able to vibrate at a lower frequency when stimulated by sound, like a mountain that would make an echo that sounds lower than your own voice?

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    Nonlinear structures have this behavior. – nicoguaro Jun 02 '20 at 00:02
  • Would you share examples of such nonlinear acoustic structures? – adrienlucca.net Jun 02 '20 at 00:25
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    In a sense, an overdriven speaker does this. When the membrane can't travel any farther, it stops abruptly, turning a sine wave into something like a square wave. These have higher frequency Fourier components. – mmesser314 Jun 02 '20 at 03:06
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    Musical instruments are often designed to produce harmonics. For example the thickness and mechanical properties of the top and bottom plates of a violin are important because of this. But it isn't clear that the violin string vibrates in a pure sine wave. The violin cavity might be picking out frequencies that are already present. – mmesser314 Jun 02 '20 at 03:09
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    As mentioned by @mmesser314, musical instruments are designed to have some sort of response in overtones (not necessarily harmonic). Although, I would not say that this is the same since different parts of an acoustic musical instrument act as a filter (see this answer). – nicoguaro Jun 02 '20 at 15:24

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There are nonlinear systems that present a sub-harmonic response. That is, a harmonic with a frequency lower than the excitation frequency. For example, a spring with a cubic nonlinearity can present this type of behavior.

There are also systems that present multi-stability, where you have different stable configurations with different values of energy (see reference 1). That is, we have an energy landscape with several local minima. So, I think that you combine both of these things to get a phenomenon similar to fluorescence if you have a structure with some local minima with higher values than others.

References

  1. Shan, S., Kang, S. H., Raney, J. R., Wang, P., Fang, L., Candido, F., ... & Bertoldi, K. (2015). Multistable architected materials for trapping elastic strain energy. Advanced Materials, 27(29), 4296-4301.
nicoguaro
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